1. Field of the Invention
The present invention relates to a method of manufacturing a perfluorinated polyfunctional vinyl ether compound.
2. Description of the Related Art
Fluorine-containing polymers each have many excellent properties, such as water- and oil-repellent properties, heat resistance, chemical resistance and having a low refractive index, and are used in a wide variety of fields, such as those of coating agents (as described, e.g., in JP-A-2006-38438), seal members (as described, e.g., in JP-A-2007-146096) and cosmetics (as described, e.g., in JP-A-2007-269642).
Among them, a perfluorinated vinyl ether compound having a perfluorinated vinyl ether group (—OCF═CF2) is one of monomers most frequently used as raw materials of fluoropolymers. A known method for manufacturing a perfluorinated vinyl ether compound on an industrial scale is decarboxylation by heating of a carboxylate containing a group —OCF2CF2COOM or a group —OCF(CF3)COOM (wherein M represents a metal, e.g., an alkali metal) (as described, e.g., in JP-A-2004-18424). However, the decarboxylation is generally attended with difficulty in drying the carboxylate. And it is known that insufficient drying of the carboxylate gives rise to formation of the group —OCHFCF3 as a by-product that comes from addition of hydrogen fluoride to the group —OCF═CF2. Presence of the group —OCHFCF3 in a compound having two or more perfluorinated vinyl ether groups is undesirable, because the group —OCHFCF3 shows no ability to polymerize and becomes a cause of a drop in cross-linkage density in the case of manufacturing a polymer by polymerizing the perfluorinated vinyl ether groups. On the other hand, dechlorination from the group —OCClFCClF2 by use of zinc (as described, e.g., in Japanese Patent No. 3,882,229) is known as another industrial method for manufacturing a perfluorinated vinyl ether compound without utilizing the decarboxylation. However, this dechlorination requires an additional operation for disposal of zinc chloride as a by-product, so it is at an economical disadvantage by incurring additional costs.
By contrast, the method of manufacturing a perfluorinated vinyl ether compound in a flow method allows continuous reaction, and is one of the most useful methods suited for industrial operations. The term “a flow method” as used herein refers to a process in which a compound having the group —OCF(CF3)COF or the group —OCF2CF2COF (a raw material) is introduced into a tubular reactor heated to high temperatures, and subjected to thermal decomposition in its vapor phase, thereby converting the group therein into the group —OCF═CF2. In the flow method, a granular filler such as glass beads is generally packed in a reactor to heighten thermal conductivity, and thereby reaction in the reactor is accelerated. And the duration of contact between a raw material and a filler in the flow method is one of important factors affecting a conversion ratio in the reaction. Therefore, for the purpose of controlling the duration of the contact, the reaction is carried out under normal atmospheric pressure while flowing an inert gas, such as nitrogen or helium, through the reactor. Moreover, the supply of an inert gas flow is important in not only drying the filler and inhibiting formation of the group —OCHFCF3 as a by-product but also accelerating the vaporization of a raw material. Therefore, all of known flow methods are performed under normal atmospheric pressure. Such a flow method can provide the desired product in a high yield so long as the compound as a raw material has one group of the formula —OCF(CF3)COF or —OCF2CF2COF. However, when it is intended to derive a compound having more than one group of the formula —OCF═CF2, namely a perfluorinated polyfunctional vinyl ether compound, from the compound having a plurality of those groups, there occurs a problem that the yield of the product is low and the productivity is bad. For instance, the yield of FSO2(CF2)2OCF(CF2OCF═CF2)2 stands at a low of 31.5% in the case of deriving such a compound from FSO2(CF2)2OCF(CF2OCF(CF3)COF)2 in accordance with a flow method(see JP-A-64-3140), so it cannot be said that the flow method has manufacturing suitability. In addition, the yield of F2C═FCOCF═CF2 stands at a low of 41% in the case of deriving such a compound from FCO(CF2)2OCF(CF3)COF in accordance with a flow method, so it is also hard to say that the flow method has manufacturing suitability.
On the other hand, where the conduct of a flow method under a reduced pressure is concerned, there are some specifications (see JP-A-2001-139509, WO 02/026682, brochure and WO 02/026687, brochure) having such a description that, when a substrate used has a high boiling point, it is advisable to conduct reaction under a reduced pressure. However, there has been no reported case of putting vapor-phase thermal decomposition reaction under a reduced pressure into practice. In addition, no description about relationship between the number of functional groups in an acid fluoride and the reaction pressure has been found, and no attempt to manufacture a perfluorinated polyfunctional vinyl ether compound(s) in a flow method under a reduced pressure has been made at all.